OCEAN-ATMOSPHERE INTERACTIONS

Eric Chassignet

Florida State University – COAPS

COUPLED OCEAN-ATMOSPHERE TIME SCALES

• S2S time scales as defined by NAS committee “14

days to 3 months”

• The ocean is often described as the flywheel of the

Earth’s climate (Visbeck et al, 2003). The ocean’s large

heat capacity (2.5 m of water contains as much thermal

energy as the entire atmospheric column) acts as

stabilizer for the Earth’s climate. The ocean reacts

more slowly than the atmosphere to changes in heat,

but also stores this heat and releases it over longer

time periods.

• Statistically, outside the tropics, the ocean has

traditionally been found to react to the high frequency

changes of the atmospheric forcing and that its

influence back to atmosphere is weak on short time

scales (Kushnir et al., 2002).

COUPLED OCEAN-ATMOSPHERE TIME SCALES

• However, one needs to distinguish between

applications that are sensitive to slowly evolving

boundary conditions (NAO, ENSO, etc.) and ones that

are sensitive to rapidly evolving boundary conditions

(diurnal cycle and MJOs, severe weather, etc.).

• Ocean-atmospheric feedback time scales that are on

the order of days occur mostly in the turbulent surface

layer (atmosphere, ocean, waves, and sea-ice) and

over the continental shelf.

• The latter is where the highest biological and human

activities are and where the majority of applications for

ocean prediction take place.

COUPLED FORECAST ERROR

A significant portion of coupled forecast error can be

directly attributed to errors in air-sea fluxes and

associated parameterizations.

From Edson et al., BAMS 2007

SURFACE ROUGHNESS OVER GULF STREAM

SAR image showing convective

cells over Gulf Stream: smooth

waters over cool water (Sikora

et al., 1995).

Photo taken over Gulf Stream

looking towards cooler shelf

waters. Courtesy P. Chang and

D. Chelton.

Slide courtesy of M. Bourassa, FSU

Loop Current

impingement

on slope

High Pressure

anomaly

extends along

shelf slope

to DeSoto

Canyon

Cross-Slope Flows in the DeSoto Canyon

Large-scale low-frequency downwelling events are 2 to 4 times more likely to occur when the Loop Current impinges on the continental slope along the southern West Florida Shelf, and upwelling events are more likely in absence of impingement.

Loop Current contact with the continental slope spawns a high pressure (SSH) anomaly that transits the continental slope toward the Mississippi Delta in the topographic Rossby wave direction. This high SSH suppresses isopycnals below, causing downwelling and/or hindering upwelling.

Nguyen et al. (2015)

Tomas et al. (2011): Impact of a high

resolution ocean in CCSM3.5

Tomas et al. (2011)

Tomas et al. (2011)

OCEAN PREDICTION SYSTEMS

• Short term ocean prediction systems have made

significant progress over the past decade.

• GODAE: Global Ocean Data Assimilation Experiment

=> Routine products at ~1/10º

• Coupled prediction systems are only starting to take

advantage of these systems [see Brassington et al.

(2015) for a review].

• Many (including NMME) typically under-sample the

ocean variability, mostly due to computational costs.

GOFS 3.0: 1/12° 32 layer HYCOM/NCODA-MVOI /MODAS synthetics/e-loan ice

• Pre-operational system that ran on Navy DSRC Cray XT5

• OPTEST report accepted by AMOP in Apr 2012

GOFS 3.01: 1/12° 32 layer HYCOM/NCODA-3DVAR/MODAS synthetics/e-loan ice

• Operational system running on Navy DSRC IBM iDataPlex computers

• Switch from MVOI to 3DVAR

• Switch from NOGAPS to NAVGEM 1.1 forcing in August 2013

• Switch from NAVGEM 1.1 to NAVGEM 1.2 forcing in March 2014

GOFS 3.1: 1/12° 41 layer HYCOM/NCODA-3DVAR/ISOP synthetics/CICE

• Add nine near surface layers

• Two-way coupled HYCOM with Los Alamos CICE model

• Replace MODAS synthetics with Improved Synthetic Ocean Profiles

• Validation test report completed

GOFS 3.5: 1/25° 41 layer HYCOM/NCODA-3DVAR/ISOP synthetics/CICE/tides

• Increase equatorial horizontal resolution to ~3.5 km

• Tidal forcing

• Scheduled to be operational in 2016-Q4

REANALYSIS 1993-2012: 1/12° 32 layer HYCOM/NCODA-3DVAR/MODAS synthetics/e-loan ice

ESPC: T359 NAVGEM, 1/12.5º HYCOM, and CICE

Items in red are different from the preceding system

GOFS and Reanalysis outputs available at http://hycom.org

U.S. NAVY GLOBAL OCEAN FORECASTING SYSTEM

MOVIE

OCEAN VALIDATION – MIXED LAYER DEPTH

GOFS 3.1

MdBE = -1.8 m

RMSE = 31.9 m

GOFS 3.0

MdBE = -3.1 m

RMSE = 36.7 m

Median

Bias

Error

2° bins

Box size

indicates

observation

count

IMPORTANCE OF AIR-SEA INTERACTIONS

• Climate/seasonal ocean models scaled down to higher resolution do not necessarily have the correct upper ocean parameterizations => need need to have metrics and increased communication with the GODAE community.

• Example: In line wind stress (Shriver et al., 2015)

The bulk parameterization for wind stress includes (Tair-SST) and 10 m winds (U10)

Usually calculated off-line on the NWP (e.g. NAVGEM) grid using NWP SST

Evidence that U10 should be replaced with (U10-Uocean)

HYCOM now has option to read in 10m winds and calculate wind stress in-line

CICE also reads in U10 for ice-atmosphere stress and gets Uocean from HYCOM for ice-ocean stress.

1/12 global 2003-2007 surface EKE (per unit mass) standard stress

formulation

stress formulation

including wind-current shear drifter observations

1/25 global HYCOM+CICE Agulhas rings

No assimilation, so expect different eddy fields

On-line has fewer rings and they follow multiple paths in the Atlantic

SSH (cm) from off-line stress

U10

SSH (cm) from in-line stress

(U10-Uocn)

COUPLED DATA ASSIMILATION

• Assimilation into a coupled model where

observations in one medium are used to generate

analysis increments in the other [minimization of a

joint cost function with controls in both media].

• Loosely (weakly) coupled: the first guess

(background) for each medium is generated by a

coupled integration.

• Reduced systems: atmosphere with corrections in

ocean mixed layer model; ocean with correction of

surface fluxes.

Rienecker (2010)

DRIVERS FOR COUPLED DATA ASSIMILATION

• Ocean analyses: inadequacy of surface fluxes from

atmospheric analyses

• Needed to reduce initialization shocks

• Needed for better surface boundary estimates for

atmospheric analyses

• Needed for an optimal representation of the

oceanic and atmospheric surface boundary layers.

SUMMARY/CONCLUSIONS

• On S2S time scales, can we computationally afford high

resolution ocean numerical models?

• Can we quantify the impact of eddy resolving ocean on

S2S time scales?

• SST representation depends on the surface mixed layer

representation and associated parameterizations.

Should a wave model be incorporated?

• Consistent coupled data assimilation is needed for an

optimal representation of the oceanic and atmospheric

surface boundary layers.

• Coupled prediction will have a significant impact on

many ocean applications (sonar prediction, search and

rescue, chemical spills, etc.)